1. Field of the Invention
The present invention relates generally to magnetic sensors and, more particularly, to a geartooth sensor which comprises a transparent plate through which a magnetically sensitive component can be calibrated by selectively trimming resistors on a semiconductor chip.
2. Description of the Prior Art
Many different types of geartooth sensors are known to those skilled in the art. Packaging techniques used in the manufacture of geartooth sensors vary significantly, based on the intended application of the sensor and the operational characteristics required for the application.
In certain magnetic sensors, two operational parameters are very important. First, the permanent magnet used in the sensor must be placed as close as possible to the target, which is generally a tooth of a gear that is positioned to move through a detection zone of the sensor. Secondly, accuracy requirements of the magnetic sensor often necessitate that internal stresses on the magnetically sensitive component be overcome to reduce the resulting offset signals and the sensor's sensitivity to thermal expansion.
It is known to those skilled in the art that magnetically sensitive components can be disposed on the pole face of a permanent magnet for applications in magnetic sensors. It is also known to those skilled in the art that magnetically sensitive components can be disposed directly on a lead assembly or other component that exhibits ferromagnetic characteristics. A ferromagnetic platform on which the magnetically sensitive component is disposed can serve as a pole piece for the sensor.
U.S. Pat. No. 5,021,736, which issued to Gonsalves et al on Jun. 4, 1991, discloses a sensor calibration method with an angular adjustment of a magnetoresistive element. The transducer arrangement comprises a magnetoresistive transducer including a magnetoresistive element which is responsive to magnetic flux passing through it in a predetermined direction to change its electrical resistance as a function of the intensity of the magnetic flux in the predetermined direction. A magnet is used to provide the magnetic flux. The transducer can include a permanent magnet having a planar surface with a magnetoresistive element offset from the axis of the magnet and optionally at an angle to the planar surface. The transducer can also include a concave surfaced magnet with the magnetoresistive element at an acute angle to the parallel flux lines emanating from the concave surface. As a further embodiment, the magnetoresistive element can be tilted on the magnet surface in two directions to provide a bias field and calibration. As a third embodiment, a high permeability pole piece can be positioned on the magnet surface between the magnet and the magnetoresistive element to minimize field variation due to nonuniformities in the magnet. There is also provided a module including an aluminum bracket mountable at the fore portion thereof and containing a magnetoresistive transducer circuit at the aft portion thereof, the circuit being coupled to a semiconductor circuit disposed at the center of the bracket. An epoxy resin encases the transducer circuit and semiconductor circuit as well as an optional cable or electrical connector at the fore portion of the bracket.
U.S. Pat. No. 5,216,405, which issued on Jun. 1, 1993 to Schroeder et al, describes a package for a magnetic field sensitive device. The package is intended for use with a magnetic field sensitive element, such as a magnetoresistor, and includes at least two planar layers of a relatively stiff ferromagnetic material which is also electrically conductive. The layers have opposed edges which are in closely spaced relation to form a narrow gap therebetween. A terminal tap is integral with and extends from each layer. A magnetic field sensitive element is on a surface of at least one of the layers and preferably extends across the gap to be on both layers. The magnetic field sensitive element has a pair of contacts each of which is electrically connected to a separate one of the layers. A permanent magnet is mounted on and insulated from the surface of the layers opposite the magnetic field sensitive element. A protective layer of an insulating material may cover the magnetic field sensitive element and the layers with the terminal tabs extending therefrom.
U.S. Pat. No. 5,341,097, which issued to Wu on Aug. 23, 1994, discloses an asymmetrical magnetic position detector. The geartooth sensor incorporates a magnet and two magnetically sensitive devices, such as Hall effect elements. The two magnetically sensitive devices are disposed in a common plane with each other with one of the devices being located at a position closer to the magnet than the other device. The common plane in which both magnetically sensitive devices are disposed is spaced apart from a central axis of the magnet by a predetermined distance. A divider is provided for determining a ratio of the magnetic field strengths that are imposed perpendicularly on the first and second magnetically sensitive devices. The ratio is used to distinguish between teeth and slots proximate the sensor wherein the common plane of the magnetically sensitive devices is generally perpendicular to the path along which the teeth and slots pass.
U.S. Pat. No. 5,444,370, which issued to Wu on Aug. 22, 1995, discloses a magnetic angular position sensor with two magnetically sensitive components arranged proximate two target tracks having complementary magnetic and nonmagnetic segments. The sensor is provided with two target tracks arranged in generally parallel association with each other. Each of the target tracks comprises magnetic and nonmagnetic segments which are arranged in alternating patterns. First and second magnetically sensitive components are disposed proximate the first and second target tracks, respectively, and a source of magnetic field is disposed proximate the first and second magnetically sensitive components. Distortions of the magnetic field imposed perpendicularly on the first and second magnetically sensitive components are used to provide first and second output signals therefrom. A third output signal, which is a function of the first and second output signals, is used to determine the location of the first and second target tracks with respect to the first and second magnetically sensitive components. The magnetic and nonmagnetic segments of the first and second target tracks can be different sizes and arranged in a pattern which permits the specific and absolute location of a movable object to be determined.
The United States Patents described immediately above are hereby explicitly incorporated by reference in this description.
In view of the known types of magnetic sensors, it would be beneficial if a means were developed for manufacturing a magnetic sensor that disposes the permanent magnet at a location as close as possible to the path along which ferromagnetic objects of a target are intended to pass. In addition, it would be significantly beneficial if a means were developed that allows a magnetic sensor to be manufactured in a manner that permits calibration to be performed after all or most manufacturing steps which might induce stresses in the magnetically sensitive components of the sensor are completed.